The present invention relates to an objective lens of an optical pickup device and to the optical pickup device, and in particular, to an objective lens wherein magnification is finite and yet the temperature characteristics are excellent for recording or reproduction for at least two optical information recording media each having a transparent base board with a different thickness and to an optical pickup device.
With regard to a recording/reproducing optical system for optical information recording media having a precision required for the conventional CD reproducing apparatus (incidentally, a recording/reproducing optical system or a recording/reproducing apparatus mentioned in the present specification includes a recording optical system, a reproducing optical system, a recording and reproducing optical system, and an apparatus employing the foregoing), an infinite conjugated optical system is disclosed in TOKKAISHO No. 57-76512, and a finite conjugated optical system is disclosed in TOKKAISHO No. 61-56314. Further, for reducing occurrence of aberration caused by a temperature change in the case of using an objective lens made of resins, those employing a coupling lens are disclosed in TOKKAIHEI No. 6-258573. However, lenses made of resin (plastic) are used widely for a recording/reproducing optical system, especially for its objective lens, because of the recent demand for low cost.
However, an objective lens made of resin materials has a problem that aberration caused by a change in a refractive index that is derived from a temperature change is greater than that of a lens made of glass materials. In general, a change of a refractive index in resin materials is different from that of a refractive index in glass materials by ten times or more. In this case, when a difference between a temperature of the standard design and a temperature in the environment used actually is represented by ΔT, aberration changed by this temperature difference ΔT is mainly tertiary spherical aberration. Let it be assumed that SA represents the tertiary spherical aberration components of wave front aberration expressed in an rms value, and a sign of SA is defined so that SA is greater than zero when the spherical aberration is positive (over), while, SA is smaller than zero when the spherical aberration is negative (under). Tertiary spherical aberration ΔSA (λrms) caused by temperature change ΔT can be expressed in the following expression by using numerical aperture NA of the objective lens on the optical information recording medium side (on the image side), focal length f, image forming magnification m, proportion coefficient k and light wavelength λ.ΔSA/ΔT=k·f(1−m)4(NA)4/λ  (1)Incidentally, when a lens made of a resin material has a positive refracting power, if a temperature rises, its tertiary spherical aberration turns out to be over. Namely, the coefficient k in the aforesaid expression takes a positive value. Further, when a single lens made of a resin material is made to be an objective lens, the coefficient k takes a greater positive value.
In the case of an objective lens used for a compact disc that is widely used presently, it can be said that aberration caused by a temperature change in the environment used does not arrive at the problematic level, because NA is about 0.45. However, optical information recording media are now promoted to be of high density.
To be concrete, there has been developed DVD (storage capacity: 4.7 GB) which is in the size mostly the same as that of CD (storage capacity: 640 MB) and has raised recording density, and it is now popularized rapidly. For reproduction of DVD, it is normal to use a laser beam with a prescribed wavelength for which a wavelength of the light source is in a range of 635-660 nm. A divergent light flux emitted from a laser light source is made to be a collimated light flux by a collimator lens generally, and then, it enters an objective lens whose NA on the DVD side is 0.6 or more to be converged on an information recording surface through a transparent base board of DVD.
In consideration of the foregoing from the viewpoint of wave front aberration, when NA, for example, is increased from 0.45 to 0.6 in the expression (1) above, wave front aberration Wrms is increased to (0.6/0.45)4=3.16 times.
Though it is considered to make focal length f to be small for the purpose of keeping the wave front aberration small based on the expression (1), in this case, it is difficult to make f to be smaller than the present value, because it is actually necessary to secure a distance of focusing operation.
With the background stated above, there have been proposed various types of objective lenses and optical pickup devices for conducting recording or reproduction, by using a single light-converging optical system, for a plurality of optical information recording media each having a transparent base board with a different thickness. It is known that the use of plastic lenses for the aforesaid objective lens and optical pickup device is advantageous for lightening a load for an actuator in the course of focusing and tracking, and for moving the objective lens rapidly, for making an optical pickup device to be light in weight, and for lowering the cost. For example, there are known an objective lens made of plastic and an optical pickup device employing the same wherein a divergent light is made to enter the objective lens for recording or reproducing of CD for restraining occurrence of spherical aberration caused by a thickness difference between transparent base boards, by utilizing that a diameter of a spot necessary for recording or reproducing for DVD (thickness of the transparent base board is 0.6 mm) and CD (thickness of the transparent base board is 1.2 mm) each having different recording density for information, is different each other and a necessary numerical aperture of the objective lens on the image side is different.
In the optical pickup device of this type, if an objective lens is made to be the finite conjugated type objective lens which is suitable for a divergent light flux from a light source to enter and an optical pickup device is made to be one employing that objective lens, for both recording or reproducing of DVD and recording or reproducing of CD, there are obtained merits that the optical pickup device can be made small and compact totally and a collimator lens to make a divergent light flux from a light source to be unnecessary. However, an objective lens which is made of plastic and satisfies various performances necessary for an optical pickup device, and an optical pickup device employing such objective lens made of plastic are not on practical use, and studies for them have not made yet.
On the other hand, in the case of a lens system using a conventional objective lens made of resin materials, there has been generated aberration that is proportional to the fourth power of numerical aperture NA of the objective lens on the image side, and is caused by refractive index change Δn of resin material derived from a temperature change, and this aberration has made it difficult to realize an objective lens and an optical pickup device both having sufficient optical performances.
With the aforesaid background, the inventors of the invention repeated trials and errors for realizing the objective lens and the optical pickup device stated above, and found out that an improvement of temperature characteristics of an objective lens is important for the realization. To be more concrete, they found out that the realization can be carried out by an objective lens and an optical pickup device, wherein there is provided a diffraction construction which makes spherical aberration for temperature changes to be satisfactory, on at least a peripheral area on at least one surface of the objective lens.
A first object of the invention is to provide a practical objective lens and an optical pickup device, wherein a divergent light emitted from a light source enters the objective lens, and sufficient properties for temperature changes in ambient conditions used are satisfied. Further, the first object of the invention is to provide a practical objective lens and an optical pickup device, wherein a divergent light emitted from a light source enters the objective lens, for a plurality of optical information recording media each having a transparent base board with a different thickness, and sufficient properties for temperature changes in ambient conditions used are satisfied, while making recording or reproducing for each information to be possible.
Further, the present invention relates to an objective lens and an optical pickup apparatus having a good temperature characteristics and a wide allowable range for a wavelength change of an light source.
An information recording surface of an optical information recording medium such as CD and DVD is usually protected by a transparent base board having a thickness stipulated by a standard. For conducting recording and reproducing for the optical information recording media, there is used an objective lens that is corrected in terms of spherical aberration for the transparent base board having that thickness. As an objective lens for recording and reproducing for these optical information recording media, various types of objective lenses are now studied, and TOKKAIHEI No. 6-258573, for example, discloses an objective lens of a refraction type wherein both sides thereof are aspheric surfaces. On this objective lens, there is introduced an aspheric surface to correct aberration of an optical system.
FIG. 52 is a diagram showing how residual aberration (spherical aberration) is generated when a thickness of the transparent base board is changed. When the spherical aberration is worsened, a diameter of a light spot formed on an information surface of an optical information recording medium is changed from the desired diameter. The desired spot diameter (range of 1/e2 of peak intensity), in this case, is approximated to Spot diameter (μm)=0.831×λ/NA, when the numerical aperture of the objective lens is represented by NA and a wavelength of the light source is represented by λ (μm). Therefore, further technologies are needed for securing interchangeability of optical information recording media each having a different thickness of the transparent base board.
TOKKAI No. 2000-81566 discloses technologies wherein spherical aberration for the specific transparent base board thickness is corrected in the wavelength used for CD or DVD, when a diffraction surface is united solidly with an aspheric surface of an objective lens. In this objective lens, over spherical aberration of base aspheric surface in a refraction system is corrected by under spherical aberration generated on the diffractive section. In this case, the diffractive section has a function to correct spherical aberration toward the under side in CD having a thick transparent base board, because the diffractive section has power that is proportional to the wavelength. Therefore, if power allocation for the refraction section and the diffractive section is properly selected, it is possible to correct spherical aberration in the transparent base board thickness of 0.6 mm for light source wavelength 650 nm in the case of using DVD and spherical aberration in the transparent base board thickness of 1.2 mm for light source wavelength 780 nm in the case of using CD. Further, TOKKAIHEI No. 11-274646 discloses an example wherein there is provided a diffraction surface which corrects fluctuations of a focus position caused by a refractive index change resulting from a temperature change of a plastic lens.
In these objective lenses, there is a tendency that a change of spherical aberration caused by temperature changes is increased as there are advanced a movement toward the finite of an optical pickup device, a movement toward a short wavelength and a movement toward high NA, for recording and reproducing for high density information. Amount of change δSA3 of 3rd order component of spherical aberration caused by temperature changes is expressed by the following expression, when NA represents a numerical aperture of an objective lens on the image side, f represents a focal length, m represents an image forming magnification and λ represents a wavelength of a laser light source.(δSA3/δT)∝f·(1−m)4·NA4/λ  (116)
Therefore, there is a tendency that temperature characteristics are deteriorated more as a movement toward an objective lens for high NA and a movement toward the finite of the objective lens are advanced, or as a movement toward a short wavelength of a laser light source is advanced. Error characteristics (conventional Example 1) in the case of designing on a conventional refracting interface are shown in “Table 14”. Incidentally, from now on (including lens data of the table), the power multiplier of 10 (for example, 2.5×10−3) is shown by the use of E (for example, 2.5×E−3).
TABLE 14δSA3 (λ rms)at DVD inTransparenttemperaturebase boardchangef (mm)thickness(δT = +30° C.,DVDmλ (nm)NA(mm)dn/dT(/° C.)δλ = +6 nm)Conventional3.007850.501.2−1.20E−04+0.011(CD)Example 1Conventional3.0−1/7.0 6500.600.6−1.20E−04+0.098Example 2Conventional3.05−1/6  6500.600.6−1.20E−04−0.002Example 3Example 13.0−1/7.0 650/7800.60/0.450.6/1.2−5.80E−06+0.002Example 23.0−1/7.0 650/7800.60/0.450.6/1.2 −1.20E−04,+0.027 +0.8E−06Example 33.0∞660/7900.65/0.450.6/1.2−5.70E−06+0.009Example 43.0∞660/7900.65/0.500.6/1.2 −1.20E−04,+0.019 +7.4E−06Example 53.0−1/7.0 650/7800.60/0.450.6/1.2 −1.20E−04,−0.004 +0.8E−06Example 63.0−1/10.0650/7800.60/0.450.6/1.2−5.80E−06+0.002Minimum pitch of ring-δSA3 (λ rms) at DVD inshaped diffractive zonewavelength change (δλ = +10 nm)Type of objective lens(μm)+0.000 (CD)Refractive surface only—+0.008Refractive surface only—+0.076Diffractive surface3δSA3 (λ rms) atDVD inMinimum pitch ofwavelengthType ofring-shapedchangeobjectivediffractive zoneDVD spotCD spot diameter(δλ = +10 nm)lens(μm)diameter (μm)(μm)+0.007Refractive—0.9031.420surface only+0.005Refractive—0.8981.414surface only+0.008Diffractive140.8461.487surface+0.032Diffractive90.8511.265surface−0.012Diffractive109.0041.359surface−0.001Diffractive88.9001.430surface
For the problems mentioned above, there is considered a method to improve temperature characteristics by employing diffraction, as shown in the prior art. However, when trying to improve temperature characteristics by employing diffraction, following two troubles are caused. First one of these troubles is that an objective lens turns out to be weak for wavelength characteristics. The direction in which spherical aberration is generated by temperature changes on a refraction section is originally different from that on a diffractive section, and when trying to improve temperature characteristics more, spherical aberration generated on the refraction section alone is canceled by strengthening effectiveness of the diffractive section relatively, but in the case of wavelength changes which are not followed by temperature changes, the aforesaid spherical aberration remains as residual aberration without being canceled, which is the reason why the objective lens turns out to be weak for wavelength characteristics.
The second trouble is that when trying to make the effectiveness of diffraction to be great, diffraction pitch becomes small and diffraction efficiency is lowered. There is a tendency, in particular, that a pitch becomes smaller as the position corresponding to the pitch moves in the direction toward the periphery of the objective lens. In the case of Conventional Example 2 in “Table 14” wherein temperature characteristics have been corrected thoroughly, a minimum pitch of the ring-shaped diffractive zone is 3 μm and diffraction efficiency is lowered to about 80% on the ring-shaped diffractive zone.
The invention is to solve the aforesaid problems, and the second object is to provide an objective lens which makes it possible to conduct recording and reproducing for optical information recording media each having a different transparent base board thickness such as DVD system (DVD-ROM and DVD+RAM) and CD system (CD-ROM and CD+RW) and an optical pickup device, while securing excellent temperature characteristics.